Organometallic cyclometallated complexes of transition metals (e.g. rhodium, iridium, platinum) have become useful materials because of their photophysical and photochemical properties. One especially important application of these compounds are as phosphorescent dopants in Organic Light-Emitting Diodes (OLEDs) because of their strong emission from triplet excited states (M. A. Baldo, et al, Appl. Phys. Letters, 75, 4 (1999)). An important class of phosphorescent cyclometallated complexes contain ligands that are at least bidentate wherein one coordination site of the ligand to the metal is through an N atom that is doubly bonded to C or another N atom, usually as part of a heterocyclic ring, and wherein another coordination site of the ligand to the metal is through a C atom. As used herein, the term “organometallic cyclometallated complex” means that at least one of the coordination sites forming the cyclic unit binding the metal atom by at least one ligand must be a metal-carbon bond. The metal-carbon bond is formed in place of a hydrogen-carbon bond of the free ligand before it is complexed. The carbon atom forming the metal carbon bond is usually also doubly bonded to another carbon as in, for example, a phenyl ring or a thienyl ring or furanyl ring. Further the carbon atom forming the metal-carbon bond also is preferably positioned so as to form a five- or six-membered metallacycle including the coordinated N atom of the ligand. A tris-cyclometallated complex has three such ligands. Some examples of iridium(III) organometallic cyclometallated complexes are shown below. 
Further, there are two isomers, facial and meridional (fac and mer), possible for such complexes having three identical but unsymmetrical bidentate ligands as illustrated below. The facial isomers are typically more desirable in OLED applications because they usually have higher quantum yields. 
It is also possible that the organometallic cyclometallating ligands are not all the same. Further, the organometallic cyclometallated complex must have at least one cyclometallating ligand forming a metal-carbon bond, but may have additional types of ligands not forming metal-carbon bonds. A common type of the latter would be complexes of the form L2MX as described in WO 02/15645 A1. Here L is a cyclometallating ligand forming metal-carbon and metal-nitrogen bonds, while X is another monoanionic bidentate ligand that does not form metal carbon bonds, such as acetylacetonate.
The usefulness and importance of organometallic cyclometallated complexes of second- and third-row transition metals have necessitated synthetic methods for preparing them more efficiently. Chassot et al., Inorg. Chem., 23, 4249–4253, (1984) have used lithiated ligands with platinum compounds that include leaving groups to form cyclometallated complexes of the ligands with platinum. Jolliet et al., Inorg. Chem., 35, 4883–4888, (1996) also used lithiated ligands to form cyclometallated complexes of the ligands with platinum or palladium, and Lamansky and Thompson, in International Patent Application WO 00/57676, used the same procedure for cyclometallated platinum complexes. These procedures suffer from low yields, as well as the relative instability of and difficulty in handling lithiated organic materials.
Organometallic cyclometallated complexes may also be formed from direct reaction of the cyclometallating ligand, wherein the carbon-hydrogen is activated and replaced by the carbon-metal bond. For example, fac-tris(2-phenylpyridinato-N,C2)iridium(III), or Ir(ppy)3, was made by reaction of 2-phenylpyridine and tris(acetylacetonate) iridium (Ir(acac)3) in glycerol solvent by K. Dedian et al, Inorg. Chem., 30, 1685 (1991). Stössel and coworkers (WO 02060910) further optimized and improved this reaction, but still using the expensive Ir(acac)3 starting material. By reacting less expensive halide complexes of Ir(III) such as iridium(III) chloride hydrate with 2-phenylpyridine in a solvent comprising a 3:1 mixture of 2-ethoxy-ethanol and water, Nonoyama obtained dimeric organometallic cyclometallated complexes such as tertakis(2-phenyl-pyridinato-N,C2′-) (di-μ-chloro)di-iridium(III). (Note: Ir(ppy)3 was later extracted as a side product in 10% yield from this reaction mixture, K. A. King, et al, J. Am. Chem. Soc., 107, 1431 (1985).) This particular solvent and the related 2-methoxy-ethanol are not desirable for practical use due to adverse health effects. M. G. Colombo, et al Inorg Chem., 33, 545 (1994), further reacted the above-cited di-iridium complex with a silver salt in neat 2-phenylpyridine to obtain Ir(ppy)3 in 75% yield. Grushin et al U.S. 2002/0190250 used this process to make additional tris-cyclometalated complexes of Ir(III) having fluorine-substitutions on phenylpyridine and phenylquinoline cyclometallating ligands. But this process requires a large excess of a ligand since it is employed as the solvent, thereby either consuming valuable material or necessitating a process to recover excess ligand.
Lamasky et al., Inorg. Chem., 40, 1704–1711, (2001) demonstrated yet another process for making tris-cyclometallated iridium complexes. First, a mixed ligand complex bis(7,8-benzoquinolinato-N,C3′)iridium(III)(acetylacetonate) was made from tetrakis(7,8-benzoquinolinato-N,C3′)(di-μ-chloro)di-iridium(III). Then the bis(7,8-benzoquinolinato-N,C3′)iridium(III)(acetylacetonate) was reacted with additional 7,8-benzoquinoline in refluxing glycerol to produce a mixture of isomers of the tris-cyclometallated complex, tris(7,8-benzoquinolinato-N,C3′)iridium(III). Kamatani et al, U.S. 2003/0068526, have also employed this reaction type for additional cyclometallated iridium complexes. But this process often yields less-desireable meridional isomers or mixtures of the facial and meridonal isomers of the tris-cyclometallated complexes. Tamayo et al., J. Am. Chem. Soc., 125, 7377–7387 (2003), have shown that reaction of dimeric organometallic cyclometallated complexes such as tetrakis(2-phenyl-pyridinato-N,C2′-) (di-μ-chloro)di-iridium(III) with sodium carbonate and additional cyclometallating ligand in glycerol can lead to formation of meridional isomers in many cases, while further reaction at higher temperatures results in formation of mostly facial isomer. However, this procedure is inconvenient for facial isomers as it necessitates finding exact conditions for the reaction of each ligand.
Copending commonly assigned U.S. Ser. No. 10/729,263 filed Dec. 5, 2003 describes a process for forming organometallic cyclometallated complexes of Ir(III) comprising the step of reacting a halide-containing complex of the metal with a silver salt and a heterocyclic organic ligand compound capable of forming an organometallic cyclometallated complex and in a solvent comprising an organic diol. However, this process fails in certain cases especially when the organometallic complex or intermediates have very low solubility in these solvents.
Despite the large number of investigations into the synthetic methodology for cyclometallated complexes, there remains a need for new methods that may provide better yields, higher purity, and control of desired isomers.